With this in mind, when you’re asking for alternative rendering engines, you’re really asking for Google to be able to get their hooks into iOS. It would be nice if Mozilla were free to develop their own engine but that’s not without downside and the most likely outcome is an iOS dominated by Chrome and ad-tracking.
The impacts go beyond iOS. The reality is that Safari is the only thing standing in the way of Google having IE-style dominance of browser usage share. As of now, Chromium has just under 80% usage share. If Chromium were as dominant as iOS as it is on desktop, it’s usage share would likely exceed 90%
Alternate engines are a fine idea in principle, but we need to consider whether a Chromium monopoly on the web is indeed an improvement from the status quo.
So yes, you're using Firefox, but you're still using the same rendering engine for your browser, as any other browser on iOS. Effectively limiting how powerful browsers can be on iOS, as Apple has full control over Webkit and doesn't allow anything else, literally anti-competitive behavior.